The present invention relates to magnetic field sensors and, more particularly, to feedback enhanced sensors capable of detecting time-varying and/or alternating magnetic fields.
Time-varying magnetic fields are often generated by a.c. mains wiring and associated distribution transformers. Occasionally, switching high current d.c. circuits or moving d.c. powered equipment and vehicles or even movement of large ferromagnetic bodies in the earth's geomagnetic field can generate time-varying magnetic fields.
Such magnetic fields may be troublesome. For example, prolonged exposure to even milligauss magnetic disturbances may result in danger to the health of humans and animals. A recent Swedish study, for example, was reported at a conference sponsored by the U.S. Department of Energy and the Electric Power Research Institute. The study indicated that children exposed to relatively weak magnetic fields from power lines near their homes develop leukemia at almost four times the expected rate. Another found that men exposed to similar magnetic field levels in the workplace had three times the expected rate of one form of leukemia.
Moreover, uncontrolled magnetic fields may adversely affect sensitive equipment. For example, electron microscope resolution has been known to deteriorate when spurious magnetic fields are present.
While certain passive shielding mechanisms, notably those fabricated from "mu-metal" materials, can isolate sensitive equipment or personnel from magnetic fields, such mechanisms are generally intended only to protect limited areas. As a practical matter, it has proven to be impossible to compensate for magnetic disturbances, at a reasonable cost, for volumes of space as great as a room, much less a building.
A key element of an a.c. magnetic field protection system is the mechanism used to detect the alternating, low frequency magnetic field. Heretofore, sensors adapted to detect a.c. magnetic fields have been limited in dynamic range and/or have been too costly and/or exhibit, across the frequency band of interest, electrical characteristics which preclude successful closed-loop operation.
Hall-effect sensors exhibit intrinsic frequency response from d.c. to many tens of megahertz, but are typically limited to a minimum field resolution of 50 nanoteslas (500 .mu.gauss) because of low output signal voltage and consequently poor signal/noise ratio. By contrast, compensation of workplace or laboratory a.c. magnetic fields implies a sensor root-mean-square (r.m.s.) noise floor at least 10 dB below the compensated field level. In systems intended for personnel protection or shielding of sensitive electronic apparatus, it is desirable that ambient fields be reduced to 10 nanoteslas (100 .mu.gauss) or less. That capability requires a sensor noise "floor" of at least 3 nanoteslas (30 .mu.gauss) for successful closed-loop negative feedback operation. Because the Hall-effect sensor noise floor is well above that level, Hall-effect devices are not useful as sensors in a high-attenuation negative feedback a.c. magnetic field compensation system.
Flux-gate and second-harmonic magnetometers typically exhibit d.c. to several kilohertz frequency response and have a noise floor of around 1 nanotesla (10 .mu.gauss), r.m.s., but such devices are relatively costly. The high cost is due to the criticality of the core material's characteristic and physical placement of the core windings, and the complexity of the associated electronic system.
Switchmode magnetometers also exhibit a constant phase shift vs. frequency (equivalent to an appreciable, fixed propagation delay) which severely limits maximum attainable stable closed-loop field attenuation in an active feedback system.
It would be advantageous to provide a feedback-enhanced sensor having low-noise performance equal to or better than that of switchmode magnetometers at a cost which is lower than Hall-effect devices with low-noise preamplification. In effect, unneeded d.c. response may be traded off for lower cost and better noise performance in the feedback-enhanced sensor.
U.S. Pat. No. 4,939,451 issued to Baran et al on Jul. 3, 1990, discloses a high power, a.c. current sensor, used to detect and measure current and power. A current shunt is provided, along with a current transformer and a low-impedance burden load. Two operational amplifiers are used in one of the embodiments to provide a current summing device.
It would be advantageous to provide a sensor for detecting a magnetic field in frequencies of an extended range between millihertz and tens of megahertz.
It would also be advantageous to provide a sensor with an adjustable time constant, so that the sensor's low frequency cutoff point can be optimally adjusted for a given coil/core combination.
It would also be advantageous to provide a sensor having a minimum number of components.
It would also be advantageous to provide a sensor having improved sensitivity, so that a biological experiment or electrical device could be protected against a.c. magnetic fields with little or no exposure thereto.